Detecting defects on the surface of wafers produced in the semi-conducting industry is a critical part of the whole production process. In one type of inspection process for such defect detection, a light beam is focused at a normal angle onto the surface, and the normal reflection from the surface is used to determine the presence of a defect. In some inspection processes, it is advantageous to inspect the surface using light that is incident at an oblique angle to the surface, and to detect the light at the oblique angle.
U.S. Patent Application 2004/0263834 to Alumot et al., whose disclosure is incorporated herein by reference, describes a method and apparatus for inspecting a surface for defects by first inspecting the surface at a high speed low resolution. Suspect areas are then inspected at a low speed high resolution. Inspections may be performed by illuminating the surface normally, and measuring returning light at an oblique angle.
U.S. Patent Application 2003/0001120 and U.S. Pat. No. 6,797,975, both to Nishiyama et al., whose disclosures are incorporated herein by reference, describe apparatus for inspecting a semiconductor device for defects. The apparatus includes an oblique illumination system, and a detector unit that receives returning light at a normal angle. The apparatus also includes auto-focus illumination and receiver units which are aligned obliquely to a surface of the device.
U.S. Pat. No. 6,856,384 to Rovira, whose disclosure is incorporated herein by reference, describes a metrology tool for measuring step heights of a sample on a surface. The tool may use normal and oblique illumination and detection of returning light, and may incorporate an auto-focus mechanism.
U.S. Pat. No. 6,853,446 to Almogy et al., whose disclosure is incorporated herein by reference, describes a variable angle inspection system. A scanning light beam may illuminate a surface at a first angle, and a deflection element may be selectively inserted into a path of the scanning beam so that the beam illuminates the surface at a second angle.
The light inspecting the wafer surface is typically maintained in focus at the surface by an auto-focus mechanism, many of which are known in the art. One such mechanism is described in U.S. Pat. No. 6,124,924 to Feldman et al., whose disclosure is incorporated herein by reference. The disclosure uses the property that in-focus light returns from a region whereon the light is focused in the form of a generally plane wave front. In contrast, out-of-focus light returns from the region in the form of a generally spherical wave front. The disclosure describes a double slit system which is configured so that when the region is in focus images of the slits align, whereas when the region is out of focus the images are misaligned.